An example of a zeolite of LTL structure type is zeolite L, and processes for the manufacture of zeolite L are described in U.S. Pat. No. 3,216,789, EP-A-219354, and EP-A-595465, the disclosures of all of which are incorporated by reference herein. The two European patent applications, which have extensive prior art discussions to which the reader is referred for more background, and the U.S. Patent list the significant X-ray diffraction data for crystalline zeolite L and give its formula in terms of moles of oxides as EQU 0.9 to 1.3 M.sub.2/n O:Al.sub.2 O.sub.3 :5.2 to 6.9 SiO.sub.2 :yH.sub.2 O,
where M represents an exchangeable cation of valence n, and y represents a value within the range of from 0 to about 9. In Proceedings of the 9th International Zeolite Conference, Ed. von Ballmoos et al, 1993, p. 297, Xianping Meng et al describe the effect of varying crystallization conditions and reactant ratios on a process for the manufacture of ultrafine (particle size about 30 nm) zeolite L.
Products of such a small particle size have advantages over larger particle size products, such as those produced by the procedure of U.S. Pat. No. 3,216,789, when used as a catalyst, or catalyst base, for reactions involving hydrocarbon conversions because of their enhanced ratio of surface area to mass, high diffusion rates and reactivities, and resistance to deactivation by pore plugging and surface contamination. For similar reasons they have advantages in hydrocarbon separations, and are also valuable as starting materials in the manufacture of supported zeolite layers, especially membranes, as described in WO 94/25151, the disclosure of which is also incorporated herein by reference. For the latter purpose, a zeolite having a particle size, whether in the form of agglomerates or single crystals, of at most 100 nm, and advantageously at most 75 nm, is normally required since the zeolite layer is formed by deposition from a colloidal suspension onto a support; if for any reason the suspension is not stable it is unsuitable for the purpose. Although EP-A-595465 describes the product of the inventive process, in which ammonia is used as a co-solvent to water in the zeolite synthesis mixture, as being in mono-crystalline form, and refers to carrying out the hydrothermal treatment at a temperature in the range of 70 to 160.degree. C. to yield a product having crystallites of diameter less than about 30 nm, it appears from the description and micrograph in the Application that the product consists of larger agglomerates of the nanocrystals incapable of forming a colloidal suspension. The same is true of the product of EP-A-323893, the small crystallites of which agglomerate into readily recoverable particles (page 2, lines 31 to 33).
It has now surprisingly been found that if a synthesis mixture as described in U.S. Pat. No. 3,216,789 is subjected to heat treatment at a temperature below 100.degree. C. a colloidal suspension of zeolite results.